Olefins have been conventionally produced from petroleum feedstocks by a number of processes including catalytic cracking of heavy oil feedstocks as well as by steam cracking of light paraffins such as ethane. Light olefins such as ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
Oxygenates, especially the lower alcohols, may be converted into light olefin(s). There are a number of technologies available for producing oxygenates including fermentation of biomass, reaction of synthesis gas derived from natural gas, petroleum liquids or carbonaceous materials including coal, recycled plastics, municipal waste or other organic materials. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Other known syngas production processes include steam reforming, autothermal reforming and combinations of these processes. Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of syngas (hydrogen, carbon monoxide and/or carbon dioxide) in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
Oxygenates, especially methanol may be converted into hydrocarbons, especially light olefins such as ethylene and propylene, by what are essentially dehydration and dehydrogenation reactions. A number of processes for converting a methanol feedstock into olefin(s), primarily ethylene and/or propylene, have been operated or proposed in the past. In general, these processes are conducted by contacting the feedstock with a molecular sieve catalyst under conditions of elevated temperature. An early example of such a process was the Methanol-to-Gasoline Process devised by Mobil Oil Corporation and implemented commercially at Motunui, New Zealand, using a fixed bed of zeolite catalyst with methanol feed produced from natural gas.
Olefins, which find wide utility as petrochemical feedstocks, may also be produced from oxygenates such as methanol. Processes for converting methanol and other oxygenates to product streams containing major amounts of olefins by reaction over zeolite catalysts are described, for example, in U.S. Pat. No. 4,025,575; U.S. Pat. No. 4,083,889 and U.S. Pat. No. 5,367,100 where the catalyst is a ZSM-5 zeolite and U.S. Pat. No. 4,471,150 and EP 0083 160 where the catalyst is an 8-membered ring zeolite such as ZSM-34. U.S. Pat. No. 4,062,905 discloses the conversion of methanol and other oxygenates to ethylene and propylene using crystalline aluminosilicate zeolites, for example Zeolite T, ZK5, erionite and chabazite; U.S. Pat. No. 4,310,440 describes the production of light olefin(s) from an alcohol using a crystalline aluminophosphate, often generically designated as AlPO4.
Some of the most useful molecular sieves for converting methanol to olefin(s) are silicoaluminophosphate molecular sieves. Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystalline framework structure of [SiO4], [AlO4] and [PO4] corner sharing tetrahedral units and may be synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent, for example, as described in U.S. Pat. No. 4,440,871. The use of SAPO molecular sieve catalysts for converting feedstocks such as methanol into olefin(s) is disclosed in U.S. Pat. No. 4,499,327, U.S. Pat. No. 4,677,242, U.S. Pat. No. 4,677,243, U.S. Pat. No. 4,873,390, U.S. Pat. No. 5,095,163, U.S. Pat. No. 5,714,662 and U.S. Pat. No. 6,166,282.
Typically, the molecular sieve crystals are formed into finished catalysts which have improved durability in the commercial conversion processes. These molecular sieve catalyst compositions are formed by combining the molecular sieve and a matrix e.g. an inorganic oxide such as alumina, titania, zirconia, silica or silica-alumina with a binder, e.g. clay, to form a coherent, stable, attrition-resistant composite of the sieve, matrix material and binder. Usually, the binder and matrix materials typically only serve to provide desired physical characteristics to the catalyst composition, and have little to no effect on conversion and selectivity of the molecular sieve.
Recently, attention has been given to the use of various materials in combination with the molecular sieve component and the matrix/binder materials to improve the performance of the catalyst in one respect or another. US 2002/0171633, for example, discloses the use of a catalyst composition comprising a SAPO, preferably SAPO-34, in combination with a calcined metal oxide such as magnesium oxide which exhibits certain acetone conversion characteristics. This combination is said to result in longer catalyst life when used in oxygenate conversion processes as well as having improved selectivity for propylene production, with reduced amounts relatively of ethane and propane.
US 2003/0176752 describes an oxygenate conversion catalyst based on a combination of a molecular sieve such as SAPO-34 with a metal oxide of Group 4 (IUPAC Periodic Table) such as zirconia or hafnia either alone or in combination with an oxide of a metal of Group 2 such as an oxide of calcium, magnesium, strontium or barium. This combination also, is said to result in extended catalyst life as well as in enhanced olefin yield and improved propylene selectivity. A similar process is described in US 2003/0176753 which describes an oxygenate conversion catalyst with similar advantages, based on a combination of a molecular sieve such as SAPO-34 with an oxide of a metal selected from Group 3 (IUPAC Periodic Table), the lanthanide or actinide series where the oxide is characterized by a defined value of carbon dioxide uptake.
US 2004/0030213 describes an oxygenate conversion catalyst based on a combination of a molecular sieve such as SAPO-34 and an oxide of a metal of Group 3, the lanthanide series and the actinide series. Examples of such oxides include lanthanum oxide, yttrium oxide, scandium oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide and thorium oxide. This catalytic combination is reported to result in similar advantages when used in methanol conversion reactions and, in addition, results in a reduction in the amounts of undesirable by-products such as aldehydes and ketones, especially acetaldehyde. It is also stated that the higher density of these catalyst compositions is believed to improve operability in the overall conversion process carried out in fluid cracking type equipment because the denser catalyst particles are retained to a greater extent within the unit, whether in the reactor of its associated regenerator, resulting in lower catalyst losses. WO 98/29370 also discloses the conversion of oxygenates to olefins over a small pore non-zeolitic molecular sieve containing an oxide of a lanthanide metal, an actinide metal, scandium, yttrium, a Group 4 metal, a Group 5 metal or combinations of these.
Another approach to the problems of achieving improved selectivity to the desired light olefins such as ethylene and propylene as well as extended catalyst life by reducing coke formation, that is, of reducing coke selectivity, is described in US 2003/0187314 which discloses the use in methanol conversion of a catalyst based on a molecular sieve such as SAPO-34 which has been treated with a solution of a metal alkyl organometallic compound in a non-proton donating solvent. Suitable metal alkyls are reported to include dimethyl zinc and exemplary solvents include light paraffins such as heptane.
These approaches have been notable but improved catalyst life as well as improved olefin yield and selectivity to the desired ethylene and propylene products is still desired.
The catalyst life is related to the coke selectivity of the catalyst since accumulation of coke on the catalyst although largely removed during regeneration will eventually result in deactivation, if only as a result of the increased exposure to hydrothermal deactivation during the regeneration. In addition, high coke laydown on the catalyst increases the demand for regeneration capacity and the regenerator is one of the most expensive items of equipment in a typical commercial methanol conversion unit. Thus, reductions in coke selectivity are highly prized.
A problem that is related to coke selectivity is that of hydrogen production: hydrogen is produced in proportion to the coke since the coke itself is formed by removal of hydrogen and oxygen components of the feedstock. The typical product stream from methanol conversion includes about 0.1 weight percent hydrogen, an amount which is low on a weight basis but corresponds to a relatively high volume (about 0.7 percent volume basis) since hydrogen is the lightest of all molecules. The volume occupied by the hydrogen in the product stream therefore requires the reactor and related equipment to be larger than they otherwise would be. Reductions in the volume of hydrogen in the conversion products therefore represents a substantial desideratum.